Re: [CAD_CAM_EDM_DRO] Knee or Quill Drive?

bwrfromuk wrote:

> Hi Group,
>
> I am in the process of converting an Enco 9x42 mill to CNC. X & Y
> axis are working fine and I recently connected drive to the knee. I
> am using 600oz-in peak servos with G320s, all axis are geared down
> 2:1. Using Master5 X and Y feed limits at 46 in/min, Z loses track at
> 3 ins/min. The torque measured to lift the Z axis is 40lb-in (belt
> off). The measured motor current (lifting)is 8 amps, measured across
> a 0.1ohm resistor with a 'scope, ripple across the 37 VDC supply is
> 2.5Vp-p. From the limited data on the motors the peak current rating
> is 12 amps max, nominal voltage is 38 volts, RPM is 900.
>
> The question I am now facing, and asking the groups advice on, is it
> best to carry on working on the knee drive or would I be better off
> somehow driving the quill?

Depending on the work planned, the only reason to power the knee is if
you will have to move greater than the quill travel in one move. If not,
then the quill is a LOT easier to move, and can be moved a lot faster,
too.

I would make a guess that the motor should be geared down much more,
as the stall ocurrs at such a low speed. You may also have much better
results with a higher voltage power supply. Much of the 38 V may be lost
in the Gecko and armature resistance. When 8 A is delivered to the motor
lifting the knee, what is the voltage across the motor terminals?

> To date I have not replaced the Z acme screw, how much improvement
> would a ball-screw give and would it add further complications like
> the knee dropping if the drive kicked out?

A fine pitch screw may not drop when unpowered, it depends on the knee
weight and the drag of the motor. A power-off brake could probably be
built very easily.

But, the real question with the current setup is what accuracy do you get?
I am assuming the knee is driven through a bevel gear arrangement like
the Bridgeport. This can lead to all sorts of cyclical errors and backlash
between the motor and the knee.

> How large a motor do I need for the measured torque? From the
> measurements and talk with Marriss I could and should improve the
> supply with additional capacitance and the supply voltage could be
> increased some, but the fundemental problem seems that the motor is
> too small. Does anyone have a source for reasonably priced brush
> servos?

40 in-lb is 640 in-Oz. So, with a 2:1 speed reduction, you should need
only 320 In-Oz. That doesn't sound like a big motor. (This is neglecting
cutting forces, which might add a significant amount.) With a 4:1
reduction, you'd only need 160 in-Oz. That is the first thing I'd try.
You could even use V-belts and pulleys just to test the performance,
and then buy the belt and sprockets after finding a ratio that works.

> What strategies do you recommend for driving the quill? I've looked
> at Jon's method of driving a dog through the stop mechanism in the
> quill and also seen a pulley on the quill coarse feed lever. Any
> others?

The coarse feed handle drives the quill through a rack and pinion.
This allows huge backlash on the quill, so it is literally a joke, unless
you'll only be using it to drill through holes. I've removed my
pinion entirely, so it won't be clunking around as the quill moves.

Jon